1. Field of the Invention
Applicant""s invention relates to an averaged guided wave inspection technology for ribbon cable.
2. Background Information
There are throttle cables in many aircraft, particularly the AO/A-10 Thunderbolt, that have the potential for failure during routine flying activities. The failure of the throttle cable can result in loss of throttle control and can lead to critical failure or loss of the aircraft. The throttle cable consists of a steel ribbon cable approximately 0.05 inch by 0.200 inch in cross-sectional dimension and ranging from approximately 26 to 32 feet in length that is contained in a stainless steel sheath casing and supported by a large number of stationary ball bearings inside the sheath. This ribbon cable directly connects the engine throttle to the throttle control in the cockpit. The stationary ball bearings allow the ribbon inside the throttle cable to move freely through the sheath. The throttle cable is strung between the cockpit and engines through the fuselage and in the process must go through several bends. An analysis of the failure modes seems to indicate that cycling the ribbon cable through these bends can lead to fatigue cracking and ultimately to failure of the ribbon. At the present time, during normal maintenance, these cables are given a force test to ensure that the ribbon cable moves freely inside the sheath. However, if the cable passes the force test there is no assurance that the ribbon does not have a defect or that the whole cable is not defective. Presently, there is no way to inspect the ribbon without the costly process of removing the entire throttle cable from the aircraft. Therefore, there is a need to develop a nondestructive evaluation technique that would allow inspection of these cables to detect any abnormalities or defects in the ribbon before failure. Since the ribbon cable is completely inaccessible to any probe except for a few inches at the end of the cable where it attaches the engine, the inspection technique must inspect the entire ribbon length from the accessible end.
The present invention accomplishes this goal and provides a nondestructive technique to inspect the throttle cables of aircraft, particularly A-10 aircraft. This inspection technique inspects the ribbon cable from its accessible end and provides complete inspection of the entire length of the ribbon by utilizing an averaged guided wave technology. In addition, this technology has application for inspection of cable in other items as well.
An ultrasonic guided wave approach was chosen that would allow an ultrasonic transducer to be placed on the accessible end of the ribbon cable and generate a guided wave that would travel down the entire length of the ribbon. This guided wave was capable of detecting small defects that would show up in the plot of reflected signal strength as a function of time (called an A-scan). However, the initial evaluation of the guided wave approach showed that the contact points between the ball bearings at the bend regions and throughout the length of the throttle cable caused reflections of guided waves as well. Since the ball bearings were spaced approximately every ⅝ inch down the length of the cable, a large number of reflected signals in the A-scan are due to the contact between ball bearings at the bend regions and the ribbon cable were observed. This caused false defect calls in addition to the masking of real defects in the bend regions. Unfortunately, the false calls were not random and simple averaging of the A-scans did not eliminate them.
However, it was found that one way to reduce the effect of these signals was to average the guided wave data collected while the ribbon was being moved back and forth through the cable sheath. This would mean that the temporal location of the reflection of the ball bearing contact with the ribbon in one waveform would be different than in the following waveforms because the ball bearings were fixed in the sheath and, as the ribbon moved inside the sheath, its relative position with respect to the ball bearings would be continually changing. In addition, the relative position of any defect in the ribbon cable stays fixed between the guided wave transducer and the defect. In these circumstances, if guided wave A-scan data is collected and averaged as the ribbon cable is being moved back and forth inside the cable sheath, then signals from the defect will continue to remain constant throughout the averaging process, but signals from the random electronic noise and signals from the ball bearing contact will be diminished because those signals are always changing in time with respect to the transducer""s position.
Applicant""s primary object for the present invention is to provide a method of inspecting throttle cables.
It is a further object of the present invention that the inspection system of the present invention gain access to the end of the throttle cable at the engine end and apply a piezoelectric transducer to the ribbon cable to generate an ultrasonic guided wave in the cable.
An additional object of the present invention is when the transducer is attached and coupled to the end of the ribbon cable, it will produce a low frequency guided wave that will propagate down the entire length of the ribbon and reflect back from any discontinuity in the cross section of the ribbon cable.
A further object of the present invention is to provide an averaging technique that allows defects to be distinguishable from other discontinuities.
Yet another object of the present invention is that in this technique, the ribbon cable with the transducer fixed on one end of it is moved to a number of positions in the cable sheath and upon generation, propagation, reflection, and reception of the wave, a waveform of the received reflections versus time is recorded to create a primary A-scan data set.
An additional object of the present invention is that the waveform received from the previous objective is recorded as the ribbon is moved inside the sheath cable.
Still another object of the present invention is for the primary and secondary A-scan data sets to then be averaged to obtain information on the position of any defects in the cable ribbon as distinguished from the ball bearing contact points.